The present invention generally relates to fabricating planar lightwave circuits that exhibit reduced polarization dependent wavelength shift. In particular, the present invention relates to providing a top cap structure over waveguides within planar lightwave circuits as a means to reduce polarization dependent wavelength shift.
As optical networks increasingly carry burgeoning Internet traffic, the need for advanced and efficient optical components rises. Optical communication systems permit the transmission of large quantities of information. Improved optical integrated circuits (OICs) are particularly needed. OICs come in many forms such as 1xc3x97N optical splitters, optical switches, wavelength division multiplexers (WDMs), demultiplexers, optical add/drop multiplexers (OADMs), and the like. Optical circuits allow branching, coupling, switching, separating, multiplexing and demultiplexing of optical signals without intermediate transformation between optical and electrical media.
Such optical circuits include planar lightwave circuits (PLCs) having optical waveguides on flat substrates, which can be used for routing optical signals from one of a number of input optical fibers to any one of a number of output optical fibers or optical circuitry. PLCs make it possible to achieve higher densities, greater production volume and more diverse functions than are available with fiber components through employment of manufacturing techniques typically associated with the semiconductor industry. For instance, PLCs contain optical paths known as waveguides formed on a silicon wafer substrate, wherein the waveguides are made from transmissive media which have a higher refractive index than the chip substrate or the outlying cladding layers in order to guide light along the optical path. PLCs are fashioned to integrate multiple components and functionalities into a single optical chip.
One important application of PLCs specifically and OICs generally involves wavelength-division multiplexing (WDM) including dense wavelength-division multiplexing (DWDM). DWDM allows optical signals of different wavelengths, each carrying separate information, to be transmitted via a single optical channel or fiber in an optical network. In order to provide advanced multiplexing and demultiplexing (e.g., DWDM) and other functions in such networks, arrayed-waveguide gratings (AWGs) have been developed in the form of PLCs.
A problem with PLCs is polarization dependence of the waveguides, typically caused by thermal stress induced waveguide birefringence. Such birefringence is experienced in varying degrees with waveguide fabrication process. The difference in thermal expansion coefficient between the waveguide top cladding layer and the substrate causes thermal stress. That stress imposed on the waveguide core in a direction parallel to the surface usually is different from that in a perpendicular direction. When the stress is asymmetric to the waveguide core, birefringence is induced undesirably rotating the optical axes.
Stress induced waveguide birefringence results in a difference of refractive index of the waveguide in the direction between parallel and perpendicular to the waveguide. The birefringence, in turn, causes polarization dependence in the waveguides, where the propagation constant for TE (transverse electric) mode is different from TM (transverse magnetic) mode. Consequently, the device characteristics change in accordance with the polarized state of the light provided to the device. For AWG device, this difference in propagation constants results in a wavelength shift in the spectral response peak or the passband of each wavelength channel. A conventional AWG may exhibit a polarization dependent wavelength shift of 0.1 nm, which is sufficient to undesirably impact the performance of a PLC containing the AWG.
One method of reducing thermal stress induced birefringence and resultant polarization dependent wavelength shift involves matching the coefficient of thermal expansion of the top cladding with the coefficient of thermal expansion of the substrate. This can be accomplished by doping the top cladding with boron, if silicon wafer is used as substrate. However, high boron concentrations in the top cladding lead to corrosion problems.
This polarization sensitivity or dependence in AWGs and other dispersive components can be minimized by bisecting the waveguides with a half waveplate, in a slot between waveguide portions. The half waveplate causes polarization swapping partway along the optical paths of the bisected waveguides, such that any input polarization samples each propagation constant equally and provides essentially no shift in peak wavelength with changes in input polarization. Thus, the spectrum for the TE and TM modes coincide through the use of the half waveplate.
There are three concerns with the use of half waveplates. First, a small fraction of the light propagating through the waveguide may be reflected back toward the input by the half waveplate, leading to unacceptably high back_reflection and directivity measurements. Thus, although the conventional use of the half waveplate reduces the polarization sensitivity problems associated with waveguide birefringence, back reflection is increased.
Second, the mere presence of a half waveplate bisecting the waveguides generates insertion loss. Insertion loss is the total optical power loss caused by the insertion of an optical component, such as a half waveplate in this instance, into an optic system. Insertion loss is expressed in dB, and is determined by the difference between the input optical power and output optical power. For example, a half waveplate bisecting the waveguides of an AWG can introduce an insertion loss of 0.5 dB.
Furthermore, the fabrication of PLCs is complicated by the extra processing steps associated with forming and precisely positioning half waveplate therein.
Polarization dependence of optical network components, such as polarization dependent wavelength shift in AWGs affect a system""s performance, especially when there are many components in the system. Consequently, there remains a need for better solutions to reduce polarization dependence in planar waveguide circuits such as AWGs, which avoid or mitigate the back-reflection problems and insertion loss problems associated with the conventional employment of half waveplates in such devices.
The following presents a simplified summary of the invention in order to provide a basic understanding of some aspects of the invention. This summary is not an extensive overview of the invention. It is intended to neither identify key or critical elements of the invention nor delineate the scope of the invention. Rather, the sole purpose of this summary is to present some concepts of the invention in a simplified form as a prelude to the more detailed description that is presented hereinafter. The present invention provides PLCs and methods for reducing waveguide birefringence and resultant polarization dependence in PLCs, especially for reducing polarization dependent wavelength shift in AWGs. The PLCs/AWGs of the present invention exhibiting reduced polarization dependent wavelength shift do so while mitigating insertion loss and/or increasing corrosion resistance.
One aspect of the invention relates to a PLC containing a bottom clad layer on a substrate, at least one waveguide on the bottom clad layer, each waveguide having a top cap layer on an upper surface thereof, and a top clad layer over the waveguides having a top cap an upper portion thereof.
Another aspect of the invention relates to a method of making a PLC involving forming a bottom clad layer on a substrate, forming a waveguide layer on the bottom clad layer, forming a top cap layer on the waveguide layer, patterning the waveguide layer and the top cap layer using a mask to form waveguides having a top cap on an upper portion thereof, and forming a top clad layer over the waveguides having a top cap an upper portion thereof.
To the accomplishment of the foregoing and related ends, certain illustrative aspects of the invention are described herein in connection with the following description and the annexed drawings. These aspects are indicative, however, of but a few of the various ways in which the principles of the invention may be employed and the present invention is intended to include all such aspects and their equivalents. Other advantages and novel features of the invention will become apparent from the following detailed description of the invention when considered in conjunction with the drawings.